The synthesis and processing of nanostructured materials, i.e., materials with grain sizes less than about 100 nm, is of great interest because such materials are known to have properties different from, and often superior to, those of conventional bulk materials. These advantages include greater strength, hardness, ductility, and sinterability, size-dependent light absorption, and greater reactivity. Applications for these advanced materials include ductile ceramics, wear-resistant coatings, thermal barrier coatings, new electronic and optical devices, and catalysts. There has been considerable progress in determining the properties of nanostructured materials, small amounts of which have been synthesized (mainly as nanosize powders), by processes such as colloidal precipitation, mechanical grinding, and gas-phase nucleation and growth. The focus of more recent research has been to produce nanostructured materials directly in a form suitable for use in a practical application, such as wear-resistant coatings. In such materials, the nanostructure is deliberately introduced to take advantage of superior properties, for example, enhanced hardness in the case of nanostructured wear-resistant coatings.
The current interest in nanostructured materials has led to the search for methods to synthesize such materials in the form of bulk solids or films. The production of nanostructured materials has generally involved two or more steps, including the controlled synthesis of nanosize powders, and the assembly of these powders into nanostructured materials by sintering or other means.
Gas-phase nucleation and growth of particles is an established route for the synthesis of nanosized powders and includes such techniques as evaporation-condensation, laser pyrolysis, and thermal plasma expansion processing. Gleiter, H., “Nanocrystalline Materials,” Prog. Mater. Sci. 33:223-315 (1989); Recknagle, K. et al., “Design and Operation of a Nanocluster Generation and Collection System,” Aerosol Sci. Technol. 22:3-10 (1995); Oda, M. et al., “Ultrafine particle films by gas deposition method,” Mat. Res. Soc. Symp. Proc. 286:121-130 (1993); Flint, J. H. et al., “Powder temperature, size and number density in laser-driven reactions,” Aerosol Sci. Technol. 5:249-260 (1986); Rao, N. et al., “Nanoparticle formation using a plasma expansion process,” Plasma Chem. Plasma Proc., 15:581-606 (1995). In many of these gas-phase processes, the nanosize powders are collected thermophoretically and consolidated in-situ using high pressure compaction to produce pellets of nanostructured materials. Gleiter, H., “Nanocrystalline Materials,” Prog. Mater. Sci. 33:223-315 (1989); Recknagle, K. et al., “Design and Operation of a Nanocluster Generation and Collection System,” Aerosol Sci. Technol. 22:3-10 (1995).
The use of inertial impaction provides a convenient route for assembling particles into consolidated materials, including nanoparticles into nanostructured materials. That is because heavy particles seeded in a light carrier gas can be deposited efficiently by accelerating the gas through a nozzle, preferably into a low pressure region, and directing the resulting high speed aerosol jet against a deposition substrate. Fernandez de la Mora, J., “Surface impact of seeded jets at relatively large background densities,” J. Chem. Phys. 82:3453-3464 (1985); Fernandez de la Mora et al., “Hypersonic impaction of ultrafine particles,” J. Aerosol Sci. 21:169-187 (1990). At sufficiently low pressures, even though the host gas may decelerate before impacting the substrate, e.g., by formation of a shock, the heavy particles continue their forward motion and impact by virtue of their greater inertia.
Until recently, high-speed impaction had been used mainly for particle measurement. More recently, however, a number of materials deposition processes based on this principle have been developed, including those that deposit heavy molecules, ultra fine particles, and large micron-sized particles. Schmitt, J. J., “Method and apparatus for the deposition of solid films of a material from a jet stream entraining the gaseous phase of said material,” U.S. Pat. No. 4,788,082 (1988); Halpern, B. L. et al., “Gas jet deposition of thin films,” Appl. Surf. Sci. 48/49:19-26 (1991); Calcote, H. F. et al., “A new gas-phase combustion synthesis process for pure metals, alloys, and ceramics,” 24th Symp. (Intl.) on Combustion, Combustion Inst. Pittsburgh 1869-76 (1992); Kashu, S. et al., “Deposition of ultrafine particles using a gas jet,” Jap. J. Appl. Phys. 23:L910-912 (1984); Oda, M. et al., “Ultrafine particle films by gas deposition method,” Mat. Res. Soc. Symp. Proc. 286:121-130 (1993); Gould, R. K. et al., “Apparatus for producing high purity silicon from flames of sodium and silicon tetrachloride,” U.S. Pat. No. 5,021,221 (1991).
Some of the inventors of this disclosure have themselves participated in developing a process, hypersonic plasma particle deposition (HPPD), for the inertial deposition of nanoparticles to form nanostructured films. In HPPD, vapor-phase reactants are injected into a thermal plasma, which is then expanded to low pressure through a nozzle. Rapid cooling in the nozzle expansion drives the nucleation of nanoparticles, which are then accelerated in the hypersonic free jet issuing from the nozzle. A substrate may be positioned normal to the flow, and particles as small as a few nanometers in diameter deposit by inertial impaction. Ballistic compaction forms a dense, nanostructured coating. In experiments involving silicon carbide deposition, the grain size observed by scanning electron microscopy (SEM), typically around 20 nm, corresponded closely to measurements by scanning electrical mobility spectrometry of the aerosol sampled in-flight downstream of the nozzle, indicating that the film retained the grain size of the impacting particles. Rao, N. P. et al., “Nanostructured materials production by hypersonic plasma particle deposition,” Nanostructured Materials, 9:129-132 (1997); Rao, N. P. et al., “Hypersonic Plasma Particle Deposition of Nanostructured Silicon and Silicon Carbide,” J. Aerosol Sci., 29:707-720 (1998); Rao, N. et al., “Plasma chem. Plasma Process 15, 581 (1995); Neumann, A. et al., “J. Nanoparticle Res., accepted for publication January 1999; Blum, J. et al., “Nanoparticle Research,” 1, 31 (1999). A patent on the HPPD process, U.S. Pat. No. 5,874,134, assigned to the Regents of the University of Minnesota, is hereby incorporated by reference.
Nanoparticle deposition processes have recently been used to create patterned films without the use of masking through the use of collimated beams of nanoparticles. Most of this work has been directed at depositing metal patterns for printed circuit board and electronics applications. Schroth, A. et al., Jpn. J. Appl. Phys., 37, 5342 (1998); Akedo, J. et al., Jpn. J. Appl. Phys., 38:5397 (1999); Akedo, M. et al., “Sensors Actuators,” A 69:106 (1998). For example, the gas jet deposition (GJD) process has been used to “write” metal patterns for depositing gold or silver particles generated by inert gas-condensation. Hayashi, C. et al., “The use of nanoparticles as coatings,” Materials Sci. Eng. A163:157-161 (1993). In this method, condensable vapor is generated above a heated crucible and particles nucleate in an inert carrier gas. The particle-laden flow then expands supersonically through a micronozzle, producing a particle beam whose dimensions are approximately the same as, or perhaps somewhat smaller than, those of the nozzle. Nozzles with inside diameters of 100 μm were used to deposit gold particles, producing tapered needle-shaped structures. Several deposition nozzle designs were tested, including rectangular slit-type geometric and multinozzle assemblies.
Similar techniques have been used to create patterned films by depositing iron nanoparticles, and more recently to fabricate high aspect ratio structures for micro-electro-mechanical systems (MEMS) applications. A variety of techniques were used, including free forming, insert-molding, and substrate masking. To suppress clogging, the nozzles were heated, so as to drive particles away from the nozzle walls by thermophoretic forces. Representative techniques are described in Kizaki, Y. et al., “Ultrafine Particle Beam Deposition I. Sampling and Transportation Methods for Ultrafine Particles,” Jpn. J. Appl. Phys. Vol. 32:5163-5169 (1993); Akedo, J. et al., “Fabrication of Three Dimensional Micro Structure Composed of Different Materials Using Excimer Laser Ablation and Jet Molding,” Proceedings of the 10th Annual International Workshop on Micro Electro Mechanical systems, Nagoya, Japan, 135-140, Jan. 26-30, 1997.
Although the GJD process has aspects similar to HPPD, the process conditions used in GJD are significantly different, with a relatively high particle source chamber pressures (−0 Torr −5 atm.), necessitating the use of light carrier gases (e.g., helium) and small nozzle dimensions (−100 μm) to achieve inertial deposition of nanoparticles. Deposition rates for GJD are on the order of 10 μm/min, with deposit dimensions close to that of the nozzle, though the sharpness of the pattern is somewhat limited by the presence of a broad “tail” surrounding the core deposit.
The GJD method is limited in its ability to produce very small features. Pattern feature dimensions have been decreased down to 40 μm by suitably reducing nozzle dimensions. Reducing the nozzle size, however, greatly diminishes the deposition rate, since the gas flow rate varies as the square of the nozzle diameter. In addition, the use of small nozzles makes it more difficult to control the nozzle-to-substrate distance precisely, which in the GJD process scales with the nozzle diameter and determines the cut-size of impacting particles. Smaller nozzles are also difficult to manufacture with precision and are more susceptible to clogging at high particle loading.
Particle beams in areas other than nanoparticles have been controlled and focused using aerodynamic lenses. Aerodynamic focusing is discussed by Dahneke, B. et al., “Similarity theory for aerosol beams,” J. Colloid Interface Sci., 87, 167-179 (1982); Fernandez de la Mora et al., “Aerodynamic focusing of particles and molecules in seeded supersonic jets,” in Rarefied Gas dynamics: Physical Phenomena, [edited by Muntz, E. P., weaver, D. P. and Campbell, D. H.], Vol. 117 of Progress in Astronautics and Aeronautics, AIAA, Washington D.C., 247-277 (1989), and aerodynamic lenses are described by Liu, P. et al., “Generating particle beams of controlled dimensions and divergence: I. Theory of particle motion in aerodynamic lenses and nozzle expansions,” Aerosol Sci. Technol. 22:293-313 (1995); Liu, P. et al., “Generating particle beams of controlled dimensions and divergence: II. Experimental evaluation of particle motion in aerodynamic lenses and nozzle expansions,” Aerosol Sci. Technol. 22:314-324 (1995). In aerodynamic focusing, particles may be shaped into a narrow beam by passing the aerosol through a series of constrictions (aerodynamic lenses). The gas undergoes a converging and diverging motion as it flows through the lenses. Due to inertia, particles seeded in the flow may either concentrate along the flow axis or be deposited on the walls of the focusing system, depending on their size. For a given lens geometry, gas composition, particle composition, and gas flow rate, this tendency to focus depends strongly on the particle size. Particles of a certain critical size are pushed to the axis, while subcritical particles show a concentrating tendency that decreases with particle size. Particles larger than critical overshoot the focal point and are removed from the flow by collision with walls. With a single lens, only particles within a very narrow range of particle sizes are focused. Particles within a broader size range can to be focused by using a set of focusing lenses with gradually decreasing diameters. Typically, five lenses may be needed to cover one order of magnitude in particle size. The lenses may be used in combination with a downstream supersonic nozzle to accelerate and deposit particles in a specified range of particle sizes. Aerodynamic focusing is discussed in U.S. Pat. No. 5,270,542, incorporated herein by reference.